The present invention relates to a capacitor, and more particularly to a solid electrolytic capacitor using an electrically conductive polymer and a method of forming the same.
The solid electrolytic capacitor has an anode, a dielectric film on surfaces of the anode, and a cathode on an outer surface of the dielectric film, so that the cathode is electrically separated by the dielectric film from the anode. The dielectric film may comprise an oxide film. The anode may comprise a porous form of a valve action metal such as tantalum, niobium or aluminum. The cathode may have a part made of a solid electrolyte which is in contact with an entire surface of the dielectric film. The solid electrolyte serves as an electrical connection between an electrode lead and the entire surface of the dielectric film. In view of a possible reduction in resistance of the capacitor, it is, of course, preferable that the solid electrolyte has a possible high electrical conductivity. It is further required that the solid electrolyte is capable of suppressing short circuit current due to defects of the dielectric film. The solid electrolyte is capable of exhibiting a transition into an insulator upon a heat generation due to the short circuit current. Therefore, the solid electrolyte is required to have both the possible high electrical conductivity and the transition capability into the insulator upon a heat generation due to the short circuit current, for which reason manganese dioxide or TCNQ complex have been used as the solid electrolyte. Particularly when the solid electrolytic capacitor is mounted on a printed circuit board, the solid electrolytic capacitor is then subjected to a heat at 240-260.degree. C. In this case, manganese dioxide has been used because manganese dioxide is thermally stable at a high temperature of at least 240.degree. C.
Consequently, the solid electrolyte for the solid electrolytic capacitor to be mounted on the printed circuit board is required to have the high electrical conductivity, the transition capability into the insulator upon the heat generation due to the short circuit current, and the thermal stability at the high temperature of at least 240.degree. C.
Manganese dioxide have the required transition capability and thermal stability but insufficient electrical conductivity of about 0.1 S/cm. Advanced solid electrolytic capacitors are required to have much higher electrical conductivity of 10-100 S/cm.
In the above recent circumstances, available solid electrolyte satisfying the above three requirements of transition capability, high thermal stability and high electrical conductivity is specified electrically conductive polymers, for example, polypyrrole, polythiophene, and polyaniline. Developments of such solid electrolytic capacitor using electrically conductive polymers as the solid electrolyte have been active. The solid electrolytic capacitor using polypyrrole as the solid electrolyte has already been commercialized.
In Japanese laid-open patent publication No. 2-15611, it is disclosed that thiophene derivatives are polymerized with ferric compound to form polythiophene to be used as the solid electrolyte for the solid electrolytic capacitor. This polymer of thiophene derivatives is superior than a polymer of pyrrole derivatives in smaller drop of electrical conductivity in a high temperature atmosphere.
In Japanese laid-open patent publication No. 5-152169, it is disclosed that immediately after the dielectric film coating the anode has been dipped into an oxidizing agent solution with a solvent almost entirely consisting of water, then another solution including pyrrole monomers or thiophene monomers for causing polymerization reaction thereof so as to form a polypyrrole film, or a polythiophene film as the solid electrolyte. These polymers of pyrrole and thiophene are also superior in smaller drop of electrical conductivity in a high temperature atmosphere.
The above electrically conductive polymer film as the solid electrolyte coats the dielectric film which further coats the valve action metal anode. The above conventional method of forming the electrically conductive polymer film as the solid electrolyte on the dielectric film results in a formation of a thin electrically conductive polymer film that is incapable of withstanding mechanical stresses applied by expansion and shrinkage of an armored resin material which coats the electrically conductive polymer film. Particularly in the vicinity of a projecting anode lead, the electrically conductive polymer film is mechanically weak, for which reason the electrically conductive polymer film is required to have an increased strength against the mechanical stresses applied by expansion and shrinkage of the armored resin material.
Even if the number of polymerization is increased to increase the thickness of the electrically conductive polymer film as the solid electrolyte for improvement in capability of withstanding mechanical stresses applied by expansion and shrinkage of the armored resin material, antinomy problem is raised with increase in equivalent series resistance of the solid electrolytic capacitor, because the increase in thickness of the film laminated over the entire surface of the valve action metal anode results in increase in the equivalent series resistance of the solid electrolytic capacitor. Notwithstanding, it is extremely important for the advanced solid electrolytic capacitor to realize a possible reduction in the equivalent series resistance.
Consequently, the above conventional method of forming the electrically conductive polymer film as the solid electrolyte is engaged with the problem in antinomy to the effect that the increase in thickness of the electrically conductive polymer film for the purpose of improvement in capability of withstanding mechanical stresses applied by expansion and shrinkage of the armored resin material would result in increase in the equivalent series resistance, whilst the decrease in thickness of the electrically conductive polymer film for the purpose of reducing the equivalent series resistance would result in decrease in capability of withstanding mechanical stresses applied by expansion and shrinkage of the armored resin material.
In the above circumstances, it had been required to develop a novel solid electrolytic capacitor free from the above problems and a method of forming the same.